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High‐ Z ′ structures of organic molecules: their diversity and organizing principles
Author(s) -
Brock Carolyn Pratt
Publication year - 2016
Publication title -
acta crystallographica section b
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.604
H-Index - 33
ISSN - 2052-5206
DOI - 10.1107/s2052520616017297
Subject(s) - enantiopure drug , crystallography , molecule , intermolecular force , group (periodic table) , space group , crystal structure , simple (philosophy) , space (punctuation) , chemistry , hydrogen bond , type (biology) , physics , chemical physics , combinatorics , mathematics , x ray crystallography , quantum mechanics , computer science , diffraction , biochemistry , philosophy , epistemology , enantioselective synthesis , catalysis , ecology , operating system , biology
A list has been compiled of 284 well determined organic structures having more than four crystallographically independent molecules or formula units ( i.e. Z ′ > 4). Another 22 structures were rejected because the space group or unit cell was probably misassigned; the rate for that type of error is then only 7%. The space‐group frequencies are unusual; half the structures are in Sohncke groups, partly because the fraction of enantiopure structures of resolvable enantiomers is higher than for lower Z ′ structures. Careful investigation of the 284 structures has shown that they are very diverse; no simple classification can describe them all. Organizing principles have, however, been recognized for almost all of them. The most common features are simple modulations and hydrogen‐bonded aggregates; only 14% of the structures have neither. In 50% of the structures n molecules are related by a pseudotranslation that would be a crystallographic translation but for small molecular displacements and rotations. In 70% of the structures there are aggregates ( e.g. n ‐mers, columns or layers) held together by strong intermolecular interactions; those aggregates usually have approximate local symmetry. Because the n ‐fold modulations and the n ‐mers often have n < Z ′, 85% of the structures with Z ′ > 5 have several features that combine to give the high Z ′ value. The number of different molecular conformations is usually small, i.e. one or two in 84% of the structures. More exotic packing features, such as ordered faults and alternating layers of different types, are found in ca 30% of the structures. A very few structures are so complex that it is difficult to understand how the crystals could have formed.